A sump pump may be needed if the seasonal high water table (spring melt raises the level of the water table in the soil) is higher than the floor level of the basement. Do you have an on-site septic disposal sysem? If so, the septic system designer should have determined the elevation of the seasonal high water table. If you are on city sewer, then the contractor may be putting in the pump as a precaution. Or he knows something. Ask him directly. The sump pump in my house makes about as much noise as the burner on my heating system. I hear a faint sound on the first floor when it is pumping. An enclosure around the pump could make it quieter.
By the way, what do you mean by "conservation area"? The realtors in my area are fond of the term "conservation land". They like to wave their hands at any wetland they see and call it "conservation land" and imply that it cannot be built upon. Big surprise 5 years later when the private owner of the land decides to put in a house on the other side of the "conservation land". It must be owned by a government entity or conservation trust, in order to be truly conservation land.